Name: Summon Energy Beast
Circles: ? / Summoning
Description: Summons a creature that consumes energy, both positive and negative. It can use a breath attack that causes either. Any damage it deals to a unit (only living or undead, constructs not included), it also gains as temporary HP (up to half its max hitpoints, only half damage dealt by attack). Use of breath weapon also uses up some temporary HP.

This is a super complex creature which I'm sure in pen & paper is quite doable. I'm not sure this is a good fit for WoM.
Also, what Element Circle would a positive/negative creature fall into?